1. Field of the Invention
The present invention relates to substrate inspection technologies using scanning electron microscope equipment and electron rays. More particularly, this invention relates to an inspection technique for performing, on a substrate such as a semiconductor wafer or a reticle having a fine pattern, metrology of such fine pattern which is formed at a reverse tapered shape or within a substrate surface and for detecting defects, if any.
2. Description of the Related Art
In a scanning electron microscope (SEM), there is widely used a method for scanning an electron beam to permit it fall onto a workpiece and for detecting secondarily obtainable secondary electrons and reflected or “back-scattered” electrons to thereby obtain a scan image (also known as SEM image). Traditionally, the size measurement of an ultrafine pattern such as a semiconductor device or the like has been performed while using as an incident electron ray an electron beam with its energy ranging from several hundred of eV to several keV.
However, in such the low-acceleration SEM, while it is possible to observe those portions being directly irradiated with the electron beam, it is unable to observe shadow portions that are created by the presence of step-like differences of a workpiece surface, resulting in the lack of an ability to perform the size measurement. For example, in the prior art low-acceleration SEM, in case where an upper face 23 of an opening is less in dimension than its bottom portion 24 as shown in FIG. 2A, what can be done is merely to perform observation and measurement of the shape of the opening's upper face 23 as in a scan image shown in FIG. 2B. Accordingly, in order to measure the size of the opening bottom face 24, it was required to cut the workpiece into portions for formation of a cross-section and then observe a shape from the cross-section. Alternatively, as shown in FIG. 3A, even when an attempt is made to measure an inter-wire distance size 28 between an electrical wiring line or lead 26 on a substrate surface and a wire lead 27 buried within the substrate, what can be measured by the low-acceleration SEM is only the wire 26 on the substrate surface so that it has been impossible to measure the distance between the wires.
A method for solving this problem and for observing/measuring an internal structure of a workpiece without having to cut the workpiece is disclosed, for example, in JP-A-7-27549. A technique as taught thereby is designed to emit an electron beam 6 which has its energy capable of penetrating part of a workpiece and reaching a portion that is not exposed with respect to the incident electron beam, and then use a scan image obtained from a secondarily generated signal to perform size measurement. Using this scheme makes it possible, at the opening such as shown in FIG. 2A, to perform size measurement of the structure of the opening bottom face 22—this face becomes a shadow of the incident electron beam and, for this reason, cannot be measured by the low-acceleration SEM—and observation of the structure of the intra-substrate wiring lead 27 and its size 28 without forming a cross-section of the workpiece.
With noticeable advances in miniaturization of semiconductor devices in recent years, ultra-fine or “micro” structure measurement increases in importance. Especially, gate shapes are becoming finer and more complicated. Depending upon whether they are manufactured successfully or not, device performance and production yield are affectable significantly. Consequently, a three-dimensional measurement technique for use with such gate structures is becoming more important. For instance, in order to lessen a gate length, there is used a structure which has a gate electrode 29 with its bottom portion 30 being narrower than an upper portion 31 as shown in FIG. 4. As the low-acceleration SEM is such that only the shape of a top surface is obtainable, a size 32 of the bottom portion is not measurable.
For example, when observing the gate electrode 29 with its cross-sectional structure shown in FIG. 4 by use of the prior art low-acceleration SEM, what can be observed is merely the shape of the upper part 31 of the gate electrode as shown in a scan image of FIG. 5. Also note that in the prior art low-acceleration SEM, there was a method of performing observation by emitting an electron beam from an oblique direction to permit the electron beam to fall onto a “shadowed” portion. Unfortunately, the method for obliquely emitting the electron beam requires the image processing for recreation or reconstruction of a stereoscopic structure from the scan image thus obtained, resulting in occurrence of a problem as to deterioration of accuracy. Alternatively, in the case of a high-density pattern, it is no longer possible to irradiate the electron beam to the bottom 30 of the gate electrode due to the fact that it lies in a shadow of its neighboring pattern. This makes it impossible to perform any intended observation.
Additionally with the prior art low-acceleration SEM, it was merely possible, in a gate having an inverted taper shape, to observe only the shape of the gate electrode upper portion 31. Thus it was unable to measure the width 32 and taper angle 33 of the gate electrode.
Regarding semiconductor device manufacturing methodology, there is known a semiconductor fabrication method having the steps of forming a spacer 35 on a gate electrode 34 as shown in FIG. 6D, and thereafter performing ion implantation (referred to as implantation hereinafter) to thereby form a junction(s) in a substrate. For example, after having formed a pattern of the gate electrode 34 as shown in FIG. 6A, implantation 36 is performed with the gate electrode 34 being as a mask, thereby forming a junction 37 in the substrate as shown in FIG. 6B. Thereafter, a spacer 35 is formed on the gate electrode 34 as shown in FIG. 6C. Then, as shown in FIG. 6C, implantation 38 is carried out with the spacer 35 as a mask, thereby forming a junction 39 as shown in FIG. 6D. In a device with such the structure, precise measurement of the structure of the gate electrode 34 and spacer 35 enables judgment of whether the device is good or bad and also prediction of the performance thereof.
However, in the prior art low-acceleration SEM, as shown in FIG. 7, only the spacer 35 and the substrate 25 are observable, and what is knowable is merely the shape of a top surface of the device. It was unable to observe any relative structure of the gate electrode 34 and the spacer 35. In the prior art, in order to observe both the gate electrode 34 and the spacer 35 at a time, it was necessary to destroy part of a workpiece for formation of a cross-section and then observe it. With this method, destruction of the workpiece was inevitable. Another problem faced with this method is an inability to measure any feature quantity that determines the device performance.
On the other hand, with the technique disclosed in JP-A-7-27549, a hole-like shape with the presence of a portion that becomes the shadow of an incoming electron beam is measured. However, the invention disclosed in JP-A-7-27549 suffers from a problem which follows: it fails to offer the capability of measuring those feature quantities required for three-dimensional (3D) measurement. The feature quantities required for the 3D measurement refer to certain information necessary for the prediction of a stereoscopic structure, such as pattern height information or the like, by way of example. Accordingly, in the invention recited in the JP-A-7-27549 document, when performing 3D measurement, the hole shape is obtained, for example, by acquiring a scan image while rotating a workpiece support stage and letting an incident beam fall along an oblique direction onto a target pattern as mounted thereon. This is because any 3D structure could not be accurately calculated from the information of secondary signal intensity, although those sizes being displayed in the scan image are measurable in the prior art.
Although it is also possible to tilt the incident beam by means of a technique for slanting the stage, this raises a need for acquiring a scan image while slanting or sloping the stage. This causes a problem as to the necessity of varying the stage angle in a way pursuant to the shape of a pattern to be inspected. In addition, in case the inspection pattern is complicate in shape, the resulting scan image becomes complicated. This leads to a problem that the image analysis for obtaining a stereoscopic structure becomes more difficult.